A feature of some electric machines, for example, brushless direct current motors (BLDC) is that they contain permanent magnets mounted or embedded on the rotor. The manner in which these magnets are retained on the rotor shaft directly affects the permissible speed of rotation of the shaft because the magnets are subjected to centripetal forces that act to dislodge the magnets from their position. Various attempts have been made in prior art to retain magnets with some also considering the reduction of flux leakage between the magnets' poles. In general, there are two categories of rotor design; these are: surface mounted magnets and internally mounted or embedded magnets. In the first, the magnets are typically epoxied or mechanically fastened to flat surfaces on the circumference of the rotor's laminations. In this manner, the flux leakage between the poles of the magnets, through the laminations, is reduced or completely eliminated. These designs, however, severely restrict the speed capability of the rotor and most of the related prior art such as U.S. Pat. No. 4,179,634 to Burson, U.S. Pat. No. 5,811,908 to Iwata, et al, U.S. Pat. No. 6,548,932 to Weiglhofer, et al, U.S. Pat. No. 6,603,232 to Van Dine et al, U.S. Pat. No. 8,058,763 to Clark et al and U.S. Pat. No. 8,487,496 to Ifrim, et al, have sought to improve on mechanically retaining the magnets. Each of the aforementioned patents teaches the retention of magnets by inserting them into U-shaped channels which are then fastened or otherwise affixed to the periphery of a rotor shaft None of these magnet retention methods is relevant to the present invention which is related to the internally mounted or embedded magnet rotor designs. An example of related prior art is U.S. Pat. No. 8,772,994 to Feng, et al which teaches a rotor lamination design that seeks to reduce flux leakage between magnetic poles and maintain mechanical strength needed to retain the magnets. As such, Feng et al trade strength of the retaining features of the lamination against thin (or small cross-section) flux pathways to reduce flux leakage. Furthermore, the geometrical construction of the rotor laminations is a trade between the strength of the material (e.g., silicon or electrical steel) versus the desire to limit field losses from the embedded magnets.
FIG. 1 is one example of the prior art lamination 10 used in a brushless DC motor. In this case, the magnets with nominal thickness and width h and w, respectively, are inserted into the laminations into an array of slots 11, 12 with the same nominal thickness and width h and w, to form magnetic pole pairs that are equally spaced at angle a around the periphery of the lamination. The angle of the slots b with respect to a radial line that bisects the magnetic pole pair, are defined to optimize performance of the motor and depends on factors that are well-known in the prior art, such as, the number of pole pairs, the size of the magnets, the rotor diameter, the trade between reluctance and magnetic torque production, etc. If the slots 11, 12 in the laminations were entirely rectangular with nominal dimensions h and w, there would be significant magnetic flux leakage between the poles of the embedded magnets. To reduce this leakage, additional pockets 13, 14 are created in the lamination by removing additional material from the slots so as to form thin bridges 15, 17 near the outer diameter of the lamination. The strength of the thin-walled sections of bridges 16, 16 and 17 limits the amount of centripetal force that can be applied to the rotor shaft; this limits the speed capability of the machine and thus its performance. When the thicknesses t1, t2 and t3 (nominally, t1 and t2 are equal) are sized for strength, they do represent potential for parasitic field losses across bridges 15, 16 and 17; this lowers the efficiency of the machine. In another example of the prior art, bridge 16 is eliminated altogether and the slot for each magnet in a pole pair is open to each other in a continuous V-shape. In this arrangement, all of the forces resisting the centripetal forces on the magnets must be reacted by remaining bridges 15 and 17 which imposes a further constraint on their size.